Cell cycle regulation of p34cdc2 kinase activity in Physarum polycephalum

Fission Yeast Exo1 and Rqh1-Dna2 Redundantly Contribute to Resection of Uncapped Telomeres

The uncapping of telomeres induces a DNA damage response. In Schizosaccharomyces pombe, deletion of pot1⁺ causes telomere uncapping and rapid telomere resection, resulting in chromosome fusion. Using the nmt-pot1-aid strain, we previously reported that Pot1 shut-off causes telomere loss and chromosome fusion in S. pombe. However, the factors responsible for the resection of uncapped telomeres remain unknown. In this study, we investigated these factors and found that concomitant deletion of rqh1⁺ and exo1⁺ alleviated the loss of telomeres following Pot1 shut-off, suggesting that Rqh1 and Exo1 are redundantly involved in the resection of uncapped telomeres. We also investigated the role of Rqh1 helicase activity and found it to be essential for the resection of uncapped telomeres. Moreover, we found that Dna2 and Exo1 function redundantly in the resection of uncapped telomeres. Taken together, these results suggest that Exo1 and Rqh1-Dna2 redundantly contribute to the resection of uncapped telomeres. Therefore, our results demonstrate that nmt-pot1-aid is an important model strain to study the role of helicases and nucleases in the resection of uncapped telomeres and to improve our understanding of DNA double-strand break repair.

Transient expression of RAD51 in the late G2-phase is required for cell cycle progression in synchronous Physarum cells

The homologous recombination factor RAD51 is highly conserved. This criterion enabled us to identify a RAD51 ortholog in Physarum polycephalum. We found that the Physarum protein presents a high homology to the human protein and cross-reacted with antibodies directed against the human RAD51. Taking advantage of the natural synchrony of millions of nuclei within a single cell of Physarum, we investigated the fluctuation of the amount of the PpRAD51 throughout the cell cycle. Our results showed that in the late G2-phase, RAD51 was transiently expressed in a large quantity. Furthermore, knocking-down RAD51 in the G2-phase abolished this transient expression before mitosis and affected cell cycle progression. These results support the idea that RAD51 plays a role in the progression of the cell cycle in the late G2-phase.

cdc2 protein kinase: structure-function relationships

Activation of the cdc2 kinase in the cell cycle occurs upon binding to a regulatory subunit called cyclin. Cyclin A associates with both Cdc2 and its homologue Cdk2. The two complexes appear in S phase but cyclin A/Cdk2 is activated earlier than cyclin A/Cdc2. Several regions in Cdc2 are involved in binding cyclins A and B. Phosphorylation of cyclin/Cdk complexes ensures that the kinase activity peaks at a specific time in the cell cycle. Phosphorylation of Thr161 in Cdc2 is required for strong cyclin binding and kinase activity in vitro; its dephosphorylation is necessary for cells to exit mitosis. We have identified a novel 'Activating factor' that stimulates binding between cyclin and Cdc2 by inducing phosphorylation of Cdc2 on Thr161. We propose that Thr161 is targeted by an additional cell cycle regulatory pathway.

A PSTAIRE CDK-like protein localizes in nuclei and cytoplasm of Physarum polycephalum and functions in the mitosis

CDKs play key roles in controlling cell cycle progression in all eukaryotes. In plants, multiple CDKs are present, among which the best characterized CDKs are PSTAIRE CDKs. In this study, we carried out Western blot, immunoelectron microscopy and antibody treatment with an anti-PSTAIRE monoclonal antibody to explore the subcellular localization and functions of PSTAIRE CDKs in Physarum polycephalum. The results of western blot and immunoelectron microscopy showed that in P. polycephalum, a PSTAIRE CDK-like protein was 34 kD in molecular weight and located in both nuclei and cytoplasm. In nuclei, the protein was mainly associated with chromosomes and nucleoli. The expression of the PSTAIRE CDK-like protein in both the plasmodia and nuclei showed little fluctuation through the whole cell cycle. When treated with an anti-PSTAIRE monoclonal antibody at early S phase, the cells were arrested in S phase, and the mitotic onset of P. polycephalum was blocked for about 1 h when treated at early G2 phase. Our data indicated that the PSTAIRE CDK- like protein has a direct bearing on the mitosis.

Chromatin remodeling by cell cycle stage-specific extracts from Physarum polycephalum

Remodeling of chromatin is an essential process allowing the establishment of specific genetic programs. The slime mold Physarum polycephalum presents the attractive advantage of natural synchrony of the cell cycle in several million nuclei. Whole-cell extracts prepared at precise stages during the cell cycle were tested for the ability to induce remodeling in erythrocyte nuclei as monitored by microscopy, protamine competition assays, micrococcal nuclease digestions, and release of histone H5. Extracts derived from two specific cell cycle stages caused opposite types of changes in erythrocyte nuclei. An increase in chromatin compaction was imparted by extracts prepared during S-phase while extracts harvested at the end of G2-phase caused increases in nuclear volume, DNA accessibility, and release of linker histone. We also found that late G2 extracts had the ability to alter the DNase I digestion profile of mononucleosomes reconstituted in vitro in a classical nucleosomes remodeling assay. The relevance of these finding to the Physarum cell cycle is discussed.

Phosphorylation of microtubule-associated proteins from the ovaries of hemipteran insects by MPF and MAP kinase: possible roles in the regulation of microtubules during oogenesis

Nutritive tubes that link the developing oocytes to the nurse cells in ovarioles of hemipteran insects contain extensive arrays of microtubules. These are established, then later depolymerised, by developmentally regulated processes. Breakdown of the microtubules corresponds with the activation of M-phase promoting factor (MPF) and mitogen-activated protein kinase (MAP kinase), later in oogenesis, as the oocytes proceed to arrest at the first meiotic metaphase [Lane and Stebbings, Roux's Arch Dev Biol 205:150-159 (1995)]. The mechanisms that lead to the breakdown of nutritive tube microtubules are unknown. Here, we have investigated the possibility that the insect ovarian microtubules are regulated by MPF- or MAP kinase-dependent phosphorylation, focusing upon the prominent high molecular weight microtubule-associated protein (HMW MAP) enriched in this system, which is a potential target for protein kinase activity in vivo. We have purified the prominent HMW MAPs from the ovaries of two species of hemipterans, and have shown them to be substrates in vitro for the activities of MPF and MAP kinase. However, although the catalytic component of MPF (p34cdc2) is present within microtubule-rich portions of hemipteran ovarioles, we have found that neither this protein nor its regulatory partner (cyclin B) co-purify with microtubules during taxol-mediated microtubule isolation.

Replication, expression and segregation of plasmid-borne DNA in genetically transformed malaria parasites

To fully exploit the transfection technology developed for Plasmodium we investigated the features of replication, expression and segregation of an episomally maintained DNA construct during a sexual blood stage development in genetically transformed parasites of P. berghei. Using DNA in situ hybridisation techniques we were able to show that the introduced DNA construct is located in the nucleus of the parasite and is not segregating uniformly during schizogony. Replication of the construct mainly takes place between 16 and 24 h after invasion of the merozoites, coinciding with chromosomal replication. Furthermore the plasmid-borne DHFR/TS gene is constitutively transcribed throughout the asexual blood stage development. Hence the DHFR/TS promoter would appear to be a useful tool in the study of (over)expression of introduced genes and performing complementation studies in transfected parasites during the complete a sexual blood stage development of P. berghei.

Interaction between the cell-cycle-control proteins p34cdc2 and p9CKShs2. Evidence for two cooperative binding domains in p9CKShs2

A universal intracellular factor, the 'M-phase-promoting factor' (MPF), displaying histone H1 kinase activity and constituted of at least two subunits, p34cdc2 and cyclin Bcdc13, triggers the G2----M transition of the cell cycle in all organisms. The yeast p13suc1 and p18CKS1 subunits and their functionally interchangeable human homologues, p9CKShs1 and p9CKShs2, directly interact with p34cdc2 and may actually be part of the MPF complex. We have chemically synthesized p9CKShs2 and several of its peptide domains in order to investigate the binding of p9CKShs2 and p34cdc2. Several arguments support the hypothesis that the N-terminal half (peptide B) and the C-terminal half (peptide E) each contain a p34cdc2-binding site and that these two binding domains cooperate in establishing a stable p9CKShs2-p34cdc2 complex: (a) only the combination of peptides B + E, and not B or E alone, is able to elute the cdc2 kinase from p9CKShs1-Sepharose beads; (b) only immobilized peptides B + E, and not immobilized B or E, bind the cdc2 kinase; (c) only the peptides B + E combination, and not B or E alone, can compete with p9CKShs1 for cdc2 kinase binding; (d) only when supplemented with E or B free peptide does the cdc2 kinase bind to B- or E-Sepharose beads, respectively. No binding occurs in the absence of free peptide. This additivity cannot be attributed to the formation of a B-E complex mimicking the full-length p9CKShs2. The cyclin B subunit is not required for the formation of the p9CKShs2-p34cdc2 complex through these two binding domains. The implications of the existence of two cooperative p34cdc2-binding domains in p9CKShs2 on the structure of the active M-phase-specific kinase is discussed.

Controlling cell cycle progress in the fission yeast Schizosaccharomyces pombe

The suitability of fission yeast as a model for understanding the eukaryotic cell cycle has been validated in five years of exciting developments. We review recent advances in understanding the nature of the controls that regulate progression through the cell cycle and the coordination of DNA replication and mitosis.

Distinct nuclear and spindle pole body population of cyclin-cdc2 in fission yeast

Cyclins, as subunits of the protein kinase encoded by the cdc2 gene are major controlling elements of the eukaryotic cell cycle. The fission yeast Schizosaccharomyces pombe has a B-type cyclin, which is a nuclear protein encoded by the cdc13 gene. Here we demonstrate the presence of two spatially distinct cdc13 cyclin populations in the nucleus of S. pombe, one of which is associated with the mitotic spindle poles. Both populations colocalize with the product of the cdc2 gene (p34cdc2). Treatment of cells with the antimicrotubule drug thiabendazole prevents cyclin degradation and blocks the tyrosine dephosphorylation and activation of cdc2. These results suggest a key regulatory role of the cdc2-cyclin complex in the initiation of mitotic spindle formation and also that mitotic microtubule function is required for cdc2 activation.